Method for cleaning of combustion gas containing impurities

ABSTRACT

A method is provided for cleaning a combustion exhaust gas containing impurities, such as sulfur oxides and hydrochloric acid, with an additive, such as calcium hydroxide, in a reactor. The combustion gas is passed into a wetting zone where water is injected into the gas. The wet combustion gas is then passed through additive injection zone where the additive is co-currently injected into the combustion gas at a location near the bottom of the injection zone. The additive injection zone is connected to the top of the wetting zone and expands conically outward from the gas discharge outlet of the wetting zone so that as the combustion gas (and additive) travel upward through the additive injection zone the velocity of the combustion gas (and additive) is decreased. The combustion gas and additive are then passed through a cylindrical section having a uniform diameter of a given height and connected to the top of the additive injection zone. The combustion gas is then discharged from the reactor and passed through a filtering unit for removal of solids.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a 371 of PCT/FI98/00258, filed Mar. 24, 1998.

FIELD OF THE INVENTION

The present invention relates to a method for cleaning of combustion gascontaining impurities in a reactor plant.

BACKGROUND OF THE INVENTION

The present invention provides that combustion gas to be cleaned issupplied to a lower part of a reactor vault, wherefrom it

is directed to flow substantially vertically upwards, and

to exit at the upper part of the reactor vault to an after-treatment orthe like, wherein

the combustion gas is wetted with a wetting agent in the first phase ofthe cleaning process of the combustion gas,

in the second phase of the cleaning process of the combustion gas atleast one additive reacting with impurities of the combustion gas,particularly with sulphur compounds and sulphur oxides, is supplied tothe combustion gas that is wetted with a wetting agent, and

in the third phase of the process, the speed of the upwards directedflow of the combustion gas that has been treated with a wetting agentand an additive is retarded in the retardation zone of the reactor vaultby enlarging the horizontal cross-section area of the reactor vault inthe flow direction of the combustion gas.

A method of the above-mentioned kind is known from publicationEP-121431. In the method of the publication the supply of combustiongases is carried out to the lower part of the reactor vault, whereafterwetting of combustion gases is performed and alkali used as an additiveand reacting with sulphur compounds of combustion gas is added. Thecombustion gas exits at the upper part of the reactor vault. However,publication EP 121431 presents no disclosure in what manner it isattended to in the process and the construction of the reactor vaultthat the combustion gas handled with the wetting agent and the additivereacting with impurities would have a sufficient retention time in thereactor vault. A sufficient retention time ensures for the first thatthe wetting of the combustion gas will be succeeded. Second, mixing ofthe wetted combustion gas and the additive as well as chemical reactionsrequire a certain time. Further, sufficient drying of exiting combustiongases (removal of wetting agent drops) should be secured before thecombustion gas is removed from the reactor vault.

BRIEF SUMMARY OF THE INVENTION

It is an aim of the present invention to present a method by which it ispossible to a great extent to eliminate the drawbacks of prior art in amanner that the impurities of the combustion gas exiting the reactorvault of the reactor plant, particularly sulphur oxides that havereacted with water in connection with wetting, have reacted with theadditive, and the combustion gas is free from wetting agent drops to theextent that it is possible to avoid harmful chemical additionalreactions from taking place in the after-treatment of combustion gases,particularly in a filter. For attaining this purpose, the method of thepresent invention is primarily characterised in that

in the fourth phase of the cleaning process of the combustion gases, theaverage flowing speed of the combustion gas flow is standardised tocorrespond substantially to the end speed of the combustion gas flow ofthe third phase of the process by forming in the reactor vault aretention zone having a substantially standardised horizontal crosssection after the retardation zone to form an extension to the same, andthat

the combustion gases are removed from the reactor vault after theretention zone.

By using the above-described solution, advantageously in combinationwith a filter, preferably a fibre filter, a method is obtained by whichit is possible to economically resolve simultaneous removal of sulphurcompounds and particles particularly, but not exclusively, in small andmedium sized combustion plants (1 to 50 MW). Treating combustion gascontaining sulphur oxides (SO_(x)) with alkali compounds, particularlywith calcium hydroxides is a generally known method for binding harmfulsulphur to an alkali compound. However, the reactions require that thesulphur oxides and alkali oxides, in particular calcium hydroxide, arein an aqueous solution. The structure of the reactor vault has to bedesigned in a manner that there is sufficient time also for thesereactions to take place. On the other hand, it is not sensible todischarge combustion gas containing wetting agent drops from the reactorvault, since it is advantageous in combustion plants to add to thetreatment of combustion gas a filter positioned after the reactor plantand intended for dust removal, in which filter the occurrence of acidicsulphur compounds dissolved in water is very harmful. The methodaccording to the present invention resolves the above-described problemcomplex in a manner that in the retardation zone which enlarges upwardsin the flow direction of the combustion gases brings about a sufficientretention time for the combustion gas and thus, at the same time, asufficient reaction time, since with an appropriate selection for thedrop size the retention of the wetting agent drops is longer than theretention of the combustion gas. The retention zone forming an extensionto the retardation zone ensures that the combustion gases leaving theupper part of the reactor vault do not contain wetting agent drops, soharmful sulphur compounds dissolved in water exist neither after thereactor vault in the combustion gas nor thus in the filter intended fordust removal of the combustion gas.

The accompanying dependent claims describe some advantageous embodimentsof the method in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 Vertical cross-section schematic of the reactor plant of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be illustrated in more detail in the followingspecification with reference made to the enclosed drawing presentingschematically a vertical cross section of an advantageous embodiment ofa reactor plant employing the method of the invention.

Referring to FIG. 1, a reactor plant employing a method of the inventioncomprises as its main parts a reactor vault 1 having in the lower partthereof an input channel 2 for the combustion gas and in the upper partan outlet channel 3 for the combustion gas. Input channel 2 has adiameter d₁ and output channel 3 has a diameter d₄. Further, the reactorplant comprises a supply arrangement 4 for wetting agent of thecombustion gas, said arrangement comprising a wetting agent line 5 and asupply line 6 for compressed air, these having a connection with nozzles7 in the reactor vault. Further, the lower part of the reactor vault hasa rinsing water aggregate 8, a mixer 9 with an actuator 9 a thereto, aswell as an outlet aggregate 10 for removal of a liquid phase situated onthe bottom of the reactor vault. The reactor device includes further asupply unit 11 for the additive reacting with the wetted combustion gas,said unit comprising a supply tube 12 in combination with a storagecontainer 13 for the additive. To the supply tube 12 is connected asupply arrangement for the additive, e.g. an input for compressed air14.

The reactor vault 1 is divided into three main parts, the lowermost ofwhich being formed by a wetting zone 1 a having a connection with theinput channel 2 for the combustion gas, the middle one being formed by aretardation zone 1 b and the uppermost by a retention zone 1 c connectedto the outlet channel 3 related to a conical top 15 of the reactor vault1. H₂ is the height of retention zone 1 c.

The wetting zone 1 a is formed by means of the walls of the reactorvault to have a substantially cylindrical form, wherein the wettingnozzles 7 are placed in the upper part of the wetting part to surround ahorizontal aperture 16 having a contact with the retardation zone 1 b.The retardation zone 1 b starts at the level of said aperture 16 andcontinues upwards as a conically enlarging part. As can be discovered inthe drawing, the horizontal cross-section area of the aperture 16 issmaller than the horizontal cross-section area of the wetting vault 1 a,wherein in the upper part of the wetting zone 1 a, around the aperture16 is formed an annular cam structure 16 a, having said wetting nozzles7 placed in its lower surface, wherein they are positioned covered in amanner that layers K possibly draining down the walls of the retardationzone 1 b pass the wetting nozzles 7 without damaging them when fallingto the bottom of the wetting zone 1 b to be removed via the outletaggregate 10.

In the described embodiment, the retardation zone 1 b is formed to openupwards in a conical manner, but it is substantial to the generalstructure of the retardation zone 1 b in the purpose of the inventionthat the horizontal cross-section area of the retardation zone 1 benlarges in the flow direction of the combustion gas upwards. In thecombustion gas wetted with wetting agent, the additive is suppliedcentrally at the level of the aperture 16 or directly thereabovesubstantially downstream in the flow direction of the combustion gas,i.e. upwards in a manner that the additive mixes with the wettedcombustion gas as evenly as possible.

As an extension to the retardation zone in the upper part of the reactorvault there is a retention zone 1 c, having a substantially constanthorizontal cross-section area. The retention zone 1 c is advantageouslyformed to be a cylindrical part having a diameter corresponding to theupper diameter of the conical form piece of the retardation zone 1 b,i.e. the size and form of the aperture 17. Subsequent to the retentionzone 1 c the top 15 of the reactor vault 1 is formed by a gathering conein which the outlet channel 3 for the combustion gases is coupled in amanner that it is directed preferably upwards and centrally with thecentre line of the retardation zone 1 b and retention zone 1 c.

The method operates according to the following manner. In the firstphase of the cleaning process the combustion gas that comes to thereactor vault 1 via the input channel is wetted with wetting agentparticularly with water brought about into shower form by compressedair. The size range of the water drops is medium, which means that thedrops cannot be too small, wherein the wetting (absorption) of thecombustion gases is of too brief duration the combustion gas dries toofast in the retardation zone 1 b. On the other hand, the drop sizecannot be too large, wherein the drops remain in the wetting zone anddrain or fall to its bottom not finding their way either to the aperture16 or upwards therefrom. By appropriate selection of drop size suchcomposition for the combustion gas is obtained in the entire area of theretardation zone 1 b of the reactor vault that humidity conditionsnecessary for attaining chemical reactions exist in the whole humiditymeasure h₁ of the retardation zone 1 b.

In the lower part of the retardation zone, the second phase of thecleaning process takes place, wherein to the combustion gas wetted withthe wetting agent a powder like additive is supplied from theadditive-supply unit 11, e.g. by means of compressed air. The additiveis usually a suitable alkali compound, a particularly suitable compoundis calcium hydroxide reacting with sulphur oxides and hydrocloric acidand other acidifying compounds.

In the retardation zone, the flowing speed of the combustion gas isretarded in a manner that the ratio V_(SA)=the initial average speed ofcombustion gas/V_(SL)=the final average speed is 10 to 40. This loweringof flowing speed of the combustion gas in the retardation zone results,in most practical embodiments where the retardation zone 1 b is conical,in the fact that an upwards opening conical angle of the conical formpiece of the retardation zone is 10° to 20°, preferably about 15°,wherein the definition formula used for the angle and/or the height h₁of the retardation zone 1 b is $\begin{matrix}{{{\tan \quad \alpha} = \frac{( {d_{3} - d_{2}} )}{2 \cdot h_{1}}},} & (1)\end{matrix}$

wherein in the formula

α=upwards opening conical angle,

h₁=height of the conical retardation zone 1 b,

d₂=diameter of the aperture 16 in the lower part of the retardation zone1 b, and

d₃=diameter of the aperture 17 in the upper part of the retardation zone1 b.

The retarding flow of the wetted combustion gas that has been treatedwith an additive takes place in the retardation zone 1 b, upwards,wherein a long retention time is ensured for the combustion gases andthus, at the same time, a long reaction time for the reactions takingplace in moist conditions required between the additive and theimpurities, mainly sulphur oxides and possibly hydrochloric acid. At thesame time the size of the drops is decreasing due to drying and in theretardation zone 1 b, in the upwards moving combustion gas flow, thedrop size is decreasing.

The final drying of wetting agent drops in the combustion gas takesplace in the retention zone 1 c, wherein the combustion gas leaving theoutlet channel 3 contains no wetting agent drops. Significant layers Kare not generated in the walls of the retention zone. In case these aregenerated, they drain downwards along the walls of the retardation zone1 b and fall on the bottom of the reactor vault. Further, the drainingslows down the downwards directing speed of the layers to the extentthat they do not cause harm when falling on the bottom of the reactorvault 1.

The diameters of the apertures 16, 17 in the bottom part and upper partof the retardation zone 1 b are defined in the formulas: $\begin{matrix}{{d_{2} = 0},{075 \cdot \sqrt{\frac{m \cdot ( {273 + \theta_{1}} )}{p \cdot ( {{\pi + 0},{0628 \cdot D_{50}}} )}}},\quad {and}} & (2) \\{{d_{3} = 0},{075 \cdot \sqrt{\frac{m}{p} \cdot ( {273 + \theta_{2}} ) \cdot ( {0,{7692 + \frac{\theta_{1} - \theta_{2}}{1718 + {1.299 \cdot \theta_{2}}}}} )}},} & (3)\end{matrix}$

wherein

d₂=diameter of the aperture (16) in the lower part of the retardationzone (1 b),

d₃=diameter of the aperture (17) in the upper part of the retardationzone (1 b),

m=mass flow (kg/s) of the combustion gas,

p=total pressure (bar) of the combustion gas,

θ₁=initial temperature (° C.) of the combustion gas,

θ₂=end temperature (° C.) of the combustion gas,

D₅₀=median diameter (μm) of the drops.

The height of the retention zone 1 c is defined by using the formula$\begin{matrix}{{h_{2} = \frac{4 \cdot {G2} \cdot {G3}}{\pi \cdot d_{1}^{2}}},} & (4)\end{matrix}$

wherein the terms G2 and G3 are defined in the following manner:$\begin{matrix}{{{F1} = {\frac{{\pi \cdot d_{2}^{2}} + {\pi \cdot d_{1}^{2}}}{8} \cdot h_{1}}},} & (5) \\{{{F2} = \frac{\theta_{1} - \theta_{2}}{\ln \frac{\theta_{1}}{\theta_{2}}}},} & (6) \\{{{F3} = {\frac{m}{p} \cdot 2}},{858 \cdot ( {273 + {F2}} ) \cdot 10^{- 3}},} & (7)\end{matrix}$

 G1=F1/F3  (8),

G2=5.0−G1  (9),

$\begin{matrix}{{{G3} = {{F3} \cdot \frac{( {273 + \theta_{2}} )}{( {273 + {F2}} )}}},} & (10)\end{matrix}$

wherein in the formulas

h₁=height of the conical retardation zone (1 b),

d₂=diameter of the aperture (16) in the lower part of the retardationzone (1 b),

d₃=diameter of the aperture (17) in the upper part of the retardationzone (1 b),

m=mass flow (kg/s) of the combustion gas,

p=total pressure (bar) of the combustion gas,

θ₁=initial temperature (° C.) of the combustion gas,

θ₂=end temperature (° C.) of the combustion gas,

D₅₀=median diameter (μm) of the drops.

The cleaned combustion gas is directed to a filter arranged for dustremoval of the combustion gas, particularly to a fibre filter (notshown), where the filtering of particulate dust of the combustion gas,which does not contain wetting agent in drop form, takes place.

Thus, the method contains an operational entirety fulfilling thefollowing operational requirements:

I) Large wetting agent drops do not reach the retardation zone 1 b,

II) Medium size wetting agent drops join the combustion gas flow, wetthe combustion gas and confront the additive,

III) The combustion gas in the upper part of the reactor vault containsno wetting agent drops.

As to the requirements I to Ill, the requirement I is fulfilled by themeasuring formula (2) for the aperture 16 in the lower part of theretardation zone 1 b. In a corresponding manner, II is fulfilled by themeasuring formula (3) for the aperture 17 in the upper part of theretardation zone 1 b. The requirements I and II involve also the formula(1), on the basis of which, after the measuring of the apertures (i.e.d₂ and d₃), the facients of the product h₁·tan α, i.e. h₁ and α aredefined. Further, the definition formula for the height of the retentionzone 1 c fulfils the requirement III.

What is claimed is:
 1. A method for cleaning impurities from combustiongas, the method comprising the steps of: (a) providing a reactor vaultformed of walls having at least an inner surface and further comprising:(i) a wetting zone, forming the lowermost portion of said reactivevault, wherein the uppermost portion of the walls of said wetting zoneare arranged to slope inwardly forming above said walls a substantiallyhorizontal aperture having a smaller horizontal cross-sectional areathan the lower portions of said wetting zone, said inwardly slopinguppermost portion of said walls of said wetting zone are provided withnozzles mounted thereon, said nozzles provide a supply of wetting agentand compressed air, and wherein said wetting zone receives a supply ofcombustion gas, comprising impurities, provided at a lower portionthereto; (ii) a retardation zone, mounted on said wetting zone and abovesaid aperture, wherein said retardation zone is an upwardly-openingconic section, wherein the lower portion of said retardation zone isprovided with a co-axially-mounted feed tube having supplies ofcompressed air and reactive additive connected thereto, said feed tubeoriented to discharge in the upward direction; (iii) a substantiallycylindrical retention zone, mounted on said retardation zone; and (iv) atop, mounted on said retention zone, comprising a downwardly-openinggathering cone having a centrally-disposed opening, and an outletchannel coupled to said opening, and wherein any horizontalcross-section of said reactor vault is substantially circular; (b)flowing said combustion gas upwardly through said wetting zone at anaverage initial flow speed, wherein said gas flows under substantiallylaminar flow conditions; (c) wetting said combustion gas in said wettingzone; (d) contacting said combustion gas with at least one reactiveadditive by allowing said reactive additive to flow in the flowdirection of said combustion gas in said upwardly opening retardationzone, wherein the combustion gas and the additive flow through theretardation zone without contacting any turbulence generation means; (e)increasing the time said at least one reactive additive contacts saidgas by reducing the average flow speed of said combustion gas in saidretardation zone such that at the exit of said retention zone said gashas an average final flow speed lower than said average initial flowspeed; and (f) removing said gas from said reactor vault through saidtop.
 2. A method for cleaning impurities from combustion gas, accordingto claim 1, wherein said impurities comprise sulfur compounds.
 3. Amethod for cleaning impurities from combustion gas, according to claim2, wherein the reactive additive reacts with said sulfur compounds.
 4. Amethod for cleaning impurities from combustion gas, according to claim2, wherein the reactive additive reacts with said sulfur oxides.
 5. Amethod for cleaning impurities from combustion gas, according to claim1, wherein said impurities comprise sulfur oxides.
 6. A method forcleaning impurities from combustion gas, according to claim 1, whereinsaid wetting agent comprises water.
 7. A method for cleaning impuritiesfrom combustion gas, according to claim 1, wherein the reactive additivecomprises an alkali compound.
 8. A method for cleaning impurities fromcombustion gas, according to claim 1, wherein the reactive additivecomprises calcium hydroxide.
 9. A method for cleaning impurities fromcombustion gas, according to claim 1, wherein the reactive additivecomprises a powder.
 10. A method for cleaning impurities from combustiongas, according to claim 1, wherein said impurities comprise an acidiccompound.
 11. A method for cleaning impurities from combustion gas,according to claim 1, wherein said impurities comprise hydrochloricacid.
 12. A method for cleaning impurities from combustion gas,according to claim 1, further comprising the step of: retarding thespeed of said combustion gas in said retardation zone such that theratio of said average initial flow speed to said average final flowspeed is from about 10 to about
 40. 13. A method for cleaning impuritiesfrom combustion gas, according to claim 1, wherein a lowermost portionof said wetting zone is substantially cylindrical and has a firsthorizontal cross-sectional area; and said aperture has a secondhorizontal cross-sectional area smaller than said first horizontalcross-sectional area; wherein said aperture forms the bottom of saidretardation zone; and wherein said combustion gas is wetted in saidwetting zone.
 14. A method for cleaning impurities from combustion gas,according to claim 1, further comprising the steps of: (a) injectingsaid reactive additive through said feed tube substantially in thedirection of flow of said combustion gas; and (b) contacting saidcombustion gas with said at least one reactive additive substantially insaid bottom of said retardation zone.
 15. A method for cleaningimpurities from combustion gas, according to claim 1, wherein saidaperture is a first aperture at the lowermost portion of saidretardation zone and wherein the diameter of said first aperture isgiven by the equation:$d_{2} = {0.075 \cdot \sqrt{\frac{m \cdot ( {273 + \theta_{1}} )}{p \cdot ( {\pi + {0.0628 \cdot D_{50}}} )}}}$

and wherein the top of said retardation zone comprises a second aperturewherein the diameter of said second aperture is given by the equation:$d_{3} = {0.075 \cdot \sqrt{\frac{m}{p} \cdot ( {273 + \theta_{2}} ) \cdot ( {0.7692 + \frac{\theta_{1} - \theta_{2}}{1718 + {1.299 \cdot \theta_{2}}}} )}}$

wherein d₂=diameter of the aperture in the lower part of the retardationzone, d₃=diameter of the aperture in the upper part of the retardationzone, m=mass flow (kg/s) of the combustion gas, p=total pressure (bar)of the combustion gas, θ₁=initial temperature (° C.) of the combustiongas, θ₂=end temperature (° C.) of the combustion gas, D₅₀=mediandiameter (μm) of the drops.
 16. A method for cleaning impurities fromcombustion gas, according to claim 15, wherein the height of saidretention zone is given by the equation:$h_{2} = \frac{4 \cdot {G2} \cdot {G3}}{\pi \cdot d_{3}^{2}}$

wherein the terms G2 and G3 are defined by the following equations:$\begin{matrix}{{F1} = \quad {\frac{{\pi \cdot d_{2}^{2}} + {\pi \cdot d_{1}^{2}}}{8} \cdot h_{1}}} \\{{F2} = \quad \frac{\theta_{1} - \theta_{2}}{\ln \quad \frac{\theta_{1}}{\theta_{2}}}} \\{{F3} = \quad {\frac{m}{p} \cdot 2.858 \cdot ( {273 + {F2}} ) \cdot 10^{- 3}}}\end{matrix}$

 G 1=F 1/F 3 G 2=5.0−G 1${G3} = {{F3} \cdot \frac{( {273 + \theta_{2}} )}{273 + {F2}}}$

and wherein h₁=height of the conical retardation zone, d₁=diameter ofcombustion gas supply means, d₂=diameter of the aperture in the lowerpart of the retardation zone, d₃=diameter of the aperture in the upperpart of the retardation zone, m=mass flow (kg/s) of the combustion gas,p=total pressure (bar) of the combustion gas, θ₁=initial temperature (°C.) of the combustion gas, θ₂=end temperature (° C.) of the combustiongas.
 17. A method for cleaning impurities from combustion gas, accordingto claim 1, wherein said retardation zone is a conic section having anupwardly opening cone angle of from about 10° to about 20° and whereinthe geometry of said retardation zone is defined by the equation:${\tan \quad \alpha} = \frac{( {d_{3} - d_{2}} )}{2 \cdot h_{1}}$

and wherein α=upwards opening conical angle, h₁=height of the conicalretardation zone h₁, d₂=diameter of the aperture in the lower part ofthe retardation zone, and d₃=diameter of the aperture in the upper partof the retardation zone.
 18. A method for cleaning impurities fromcombustion gas, according to claim 17, wherein said upwardly openingconical angle is about 15°.
 19. A method for cleaning impurities fromcombustion gas, according to claim 1, wherein said retention zone issubstantially cylindrical, and has a diameter substantially equivalentto the diameter of the aperture in the upper part of said retardationzone.
 20. A method for cleaning impurities from combustion gas,according to claim 1, wherein said top of said reactor vault comprises:a downwardly opening conic section having an opening in the narrow endof said conic section; and an exhaust stack for said combustion gasattached to said top wherein said opening in said narrow end conductssaid combustion gas into said exhaust stack.
 21. A method for cleaningimpurities from combustion gas, according to claim 20, wherein saidopening and said exhaust stack are coaxial with said reaction vault. 22.A method for cleaning impurities from combustion gas, according to claim21, wherein said exhaust stack directs said combustion gas to a filter.23. A method for cleaning impurities from combustion gas, according toclaim 22, wherein said filter is a fiber filter.
 24. A method forcleaning impurities from combustion gas, according to claim 22, whereinsaid filter is a dust removal filter.